diff --git a/packaging_uom/models/product_packaging.py b/packaging_uom/models/product_packaging.py
index c98a2e28e..20586ee41 100644
--- a/packaging_uom/models/product_packaging.py
+++ b/packaging_uom/models/product_packaging.py
@@ -1,7 +1,8 @@
 # -*- coding: utf-8 -*-
 # Copyright 2015-2017 ACSONE SA/NV (<http://acsone.eu>)
 # License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl).
-from odoo import api, fields, models
+from odoo import api, fields, models, _
+from odoo.exceptions import ValidationError
 
 
 class ProductPackaging(models.Model):
@@ -18,9 +19,9 @@ class ProductPackaging(models.Model):
     uom_id = fields.Many2one(
         'product.uom',
         'Unit of Measure',
-        required=True,
         help="It must be in the same category than "
-             "the default unit of measure."
+             "the default unit of measure.",
+        required=False
     )
     uom_categ_domain_id = fields.Many2one(
         default=_default_uom_categ_domain_id,
@@ -28,6 +29,7 @@ class ProductPackaging(models.Model):
     )
     qty = fields.Float(
         compute="_compute_qty",
+        inverse="_inverse_qty",
         store=True,
         readonly=True
     )
@@ -43,3 +45,38 @@ class ProductPackaging(models.Model):
                 1, to_unit=self.product_tmpl_id.uom_id)
         else:
             self.qty = 0
+
+    @api.one
+    def _inverse_qty(self):
+        """
+        The inverse method is defined to make the code compatible with
+        existing modules and to not break tests...
+        :return:
+        """
+        category_id = self.product_tmpl_id.uom_id.category_id
+        uom_id = self.uom_id.search([
+            ("factor", "=", 1.0 / self.qty),
+            ('category_id', '=', category_id.id)])
+        if not uom_id:
+            uom_id = self.uom_id    .create({
+                'name': "%s %s" % (category_id.name, self.qty),
+                'category_id': category_id.id,
+                'rounding': self.product_tmpl_id.uom_id.rounding,
+                'uom_type': 'bigger',
+                'factor_inv': self.qty,
+                'active': True
+            })
+        self.uom_id = uom_id
+
+    @api.multi
+    @api.constrains
+    def _check_uom_id(self):
+        """ Check uom_id is not null
+
+        Since the field can be computed by the inverse method on 'qty',
+        it's no more possible to add a sql constrains on the column uom_id.
+        """
+        for rec in self:
+            if not rec.uom_id:
+                raise ValidationError(_("The field Unit of Measure is "
+                                        "required"))
diff --git a/packaging_uom/views/product_packaging_views.xml b/packaging_uom/views/product_packaging_views.xml
index 1f3b1dd9c..31a271220 100644
--- a/packaging_uom/views/product_packaging_views.xml
+++ b/packaging_uom/views/product_packaging_views.xml
@@ -25,7 +25,7 @@
         <field name="arch" type="xml">
             <field name="qty" position="before">
                 <field name="uom_categ_domain_id" invisible="1"/>
-                <field name="uom_id" domain="[('category_id', '=', uom_categ_domain_id)]"/>
+                <field name="uom_id" domain="[('category_id', '=', uom_categ_domain_id)]" required="True"/>
             </field>
         </field>
     </record>